The advent of wireless telephony, also referred to as cellular telephony, has given rise to a phenomenal development of new methods and technologies, many directed toward improving the quality of communication and toward accommodating more channels in the same communication spectrum. One approach is Code Division Multiple Access, or CDMA. CDMA standards are defined by IS-95.
In a CDMA system, multiple access is based on spread spectrum technology. A unique binary spreading sequence, a code, is assigned for each call to each user. Multiplied by one assigned code, the user signal is "spread" onto a bandwidth much wider than the original signal. The ratio of the two signals is commonly called the spreading factor. All active users share the same frequency spectrum at the same time. The signal of each user is separated from the others at the receiver by using a correlator keyed with the associated code signal to "despread" the desired signal. Since multiple users share the same frequency spectrum, other users' signals contribute interference. By reducing interference, CDMA system capacity can be increased.
Thus, power control is highly important. Uplink, portable or mobile to base station transmission, power control may ideally seek to control the transmit power from the portable units or mobiles within a cell so that the cell site's receivers receive the same nominal power from all the portables within the cell. If the power can be controlled perfectly, the overall interference can be minimized for the weakest users. As a result, CDMA system capacity in terms of the number of simultaneous users that can be handled in a given system bandwidth can be maximized. Further, by adding additional antenna diversity, power control is improved because fast fading is reduced.
Of course, power cannot be perfectly controlled although a variety of techniques have been employed. It is well recognized, however, that increased system capacity is highly desirable.
In a typical CDMA system, an uplink signal from a mobile unit is captured and processed utilizing a four-finger rake receiver. The rake receiver takes advantage of multipath temporal diversity from multiple antennas. Energy is gathered in the fingers from a maximum of four paths and a single output data stream is generated. Because a form of optimal combining is used to form the output data stream, this arrangement results in a significant uplink gain. The highest uplink gain is achieved when all four rake fingers are actively capturing energy.
A disadvantage of the present state of the art is that not all four rake fingers typically capture energy, but instead one or more of the rake fingers undergo periods of inactivity. Present CDMA base stations typically employ two-branch diversity antennas connected to separate RF inputs. The inputs are down-converted, digitized and assigned to one or more rake fingers. The rake fingers are intended to track two primary and two reflected paths received by two antennas. However, in practice, it is unlikely for all four fingers to be active at the same time. Typically, only two fingers are active at any given point in time. This typical level of finger activity may be caused, for example, by fading of the primary signal due to short-delay multipath fading, from reflected paths that are too weak to be tracked by the rake receiver, or the like. If all four rake fingers could be constantly supplied with energy from four or more sources, then optimal combining of the finger energy would yield a substantially increased uplink gain, and with independent fading on the branches even further gain would be achieved. With increased gain, lower transmission powers can be employed and increased system capacity achieved. Thus, a system which will help ensure the supply of a constant stream of energy to each of the four rake fingers of a CDMA system is needed and will be highly advantageous.